So far, he's snared three companies and 34 people, including Mr Trump's former national security adviser Michael Flynn, his former campaign chairman Paul Manafort and his former confidant Roger Stone.

Despite plenty of whispers, it's unknown how much longer the probe will continue.

Mr Mueller himself has remained almost silent throughout the investigation, but he's come under repeated attack from Mr Trump, who has labelled the probe a "witch hunt."

While it's unlikely to recommend any direct action against Mr Trump, Mr Mueller's final report could be used as the catalyst for further investigation by Congressional committees.

House Judiciary Committee

This committee from the lower, Democrat-controlled chamber of Congress will explore whether Mr Trump:

Obstructed justice in a "number of criminal investigations"

Violated the emoluments clause of the constitution, which says politicians can't accept gifts

Breached campaign and financial laws

Abused power in various ways, encompassing "attacks on the press", "misuse of the pardon power" and "attempts to misuse the power of the Office of the Presidency"

If you think that sounds very broad, you'd be right.

As his opening salvo, committee chairman Jerrold Nadler requested documents from 81 people and organisations connected to Mr Trump, including his two sons and son-in-law.

"This is just the first shot in an investigation that's going to take a while. They want to get their hands on the same stuff that Robert Mueller has," explained Darren Samuelsohn, senior White House reporter for Politico.

Mr Nadler confirmed this was only the first round of requests, which means this investigation could run until the 2020 presidential election.

The committee, as with all congressional committees, can demand documents and issue subpoenas to force an individual to testify. Those who violate the subpoenas could be imprisoned.

Crucially, it's this committee alone that has the power to begin any impeachment proceedings against Mr Trump.

That's a way off for now, Mr Nadler said.

Any attempt to impeach would have to get through the Senate, which is currently Republican controlled.

The Southern District Court of New York

Robert Mueller's investigation might get all the headlines.

But plenty have said the investigations being run out of the US Attorney's office in the Southern District of New York (SDNY) are where the real danger for Mr Trump lie.

The SDNY is not limited by a specific mandate, like Mr Mueller's team is. Its investigators are allowed to dig into any areas where they suspect a crime may have been committed in their jurisdiction. (The Trump campaign and Trump organisation were both run out of Trump Tower in Manhattan.)

Nick Ackerman, a former SDNY prosecutor, explained that the court has such a reputation for taking on cases the Justice Department misses, it's sometimes referred to as the "Sovereign District of New York".

"They view the Department of Justice more as an impediment than a real partner sometimes," Mr Ackerman said. "The Southern District of New York has a reputation for being independent. It's always been independent."

But they don't have to make the extent of their investigations public.

"Traditionally, this stuff is never publicised. It's all grand jury investigations which are all done in secret," Mr Ackerman said. "And if they don't find anything, you'll never know."

In his guilty plea, Cohen (who's about to report for a three-year jail sentence for his role in the payments among other things) directly implicated Mr Trump in the hush-money scheme.

The SDNY has offered immunity to the parent company of the National Enquirer and the chief financial officer for the Trump organisation, Allen Weisselberg.

That means despite the jailing of Cohen, it's likely we've not heard the last about those hush-money payments.

We also know the SDNY is looking into Mr Trump's inaugural committee and has issued subpoenas to find out exactly how it spent more than $100 million that was raised for his inauguration.

Can Mr Trump do anything to stop the SDNY's work?

"Not without getting in a lot of trouble," Mr Samuelsohn said. "He can perhaps have a say over who's overseeing things. I guess he could, ultimately, mess with their budget. But if he started to do those things, we would hear about it."

"These are career prosecutors who follow the law, no matter who the president of the United States is."

"It could end up being fodder for future obstruction of justice charges. It could spiral its way back into the impeachment process. And it could end up in the form of a sealed indictment, waiting there for the President the day that he's no longer president."

House Intelligence Committee and House Oversight Committee

They're digging into similar territory as the special counsel investigation.

Intelligence Committee chairman Adam Schiff opened an investigation into "efforts by Russia and other foreign entities to influence the US political process" in February.

The Committee also plans to investigate whether any foreign actors have financial leverage over Mr Trump, his family, his businesses or associates.

House Oversight and Reform Committee chairman Elijah Cummings has flagged numerous areas he wants to investigate, some of which arose from Michael Cohen's public testimony before the committee.

They include the security clearances given to Mr Trump's family, issues related to the Emoluments Clause of the US constitution, the handling of the travel ban and the President's refusal to release his tax returns.

The heads of the House Foreign Affairs, House Oversight Committee and House Intelligence Committees have also requested documents about Mr Trump's conversations with Russian President Vladimir Putin (the White House has pushed back, citing executive privilege).

As with the House Judiciary Committee, you should expect to hear more about these committees as they publicly request more documents and testimony in coming weeks.

"Law enforcement is looking to see if the President broke any laws in his campaign. The House of Representatives isn't looking to find lawbreakers in the same sense," Mr Samuelsohn said.

"They're looking to see if the President committed the kind of 'high crimes and misdemeanours' laid out in the constitution."

State investigations

The attorneys-general of New York, Maryland and the District of Columbia are steering three different probes.

And despite an announcement that it will be dissolved, the Trump Foundation has become a two-year focus for the New York Attorney-General.

A lawsuit alleges the foundation misused its charitable donations in service of Mr Trump's personal and political interests.

While over in the Capitol, the attorneys-general of Maryland and DC have launched a lawsuit that alleges Mr Trump is in breach of the emoluments clause of the US constitution.

The lawsuit alleges that Mr Trump is profiting from foreign governments because foreign dignitaries are more likely to stay at the Trump International Hotel because Mr Trump is the President.

In December, a judge approved 30 subpoenas directed at 13 Trump entities, with Maryland's Attorney-General telling NPR the requests aimed to establish which foreign governments are conducting business with the hotel.